Iranian students say there is a second cultural revolution under way in the universities with scores of professors forcibly retired and politically active students being threatened with expulsion.

Student anger exploded with an unprecedented show of defiance when President Mahmoud Ahmadinejad went to Tehran's Amir Kabir University on 12 December.

Pictures shot on a mobile phone showed angry students chanting against the president, accusing him of being a fascist and a puppet of the hardliners.

They held portraits of Mr Ahmadinejad upside down to mock him and then set them on fire.

The ears of the government are open to hear the student demands

Mahmoud AhmadinejadIranian President

The day before the president visited, the university was in turmoil with students shouting "Death to the dictator".

Iranian television only showed a few seconds of the disturbance. Later Mr Ahmadinejad put a brave face on it saying the protest showed there was freedom of speech in Iran compared to his student days under the Shah.

'Harassment and purges'

When Mr Ahmadinejad came to power the universities were quiet.

They [the government] have stepped up the pressure to scare students; to send a message that if you want to be politically active you will have problems in the future

Ali Nikoo NesbatiIranian activist

But by trying to stop students getting involved in politics, the new government has antagonised them.

"We think they've done this on purpose to frighten us; to send a message that if you want to be politically active you will have problems in the future," he says.

According to student activists 181 students have received letters warning them not to get involved in politics, while 47 student publications and 28 student organisations have been closed in the last year.

"They threatened me that if I talked to the media it might make things much worse for me," says Mehdi Aminzadeh, who has been banned from doing a masters in political science because he has been too active in politics.

"But if we keep silent it's easier for them to do the same things to other people," he says.

'Three-star students'

Mr Mehdi has twice been arrested and still has court cases pending against him.

Photos of the 6 December rally at Tehran University show a large crowd

He is what is known perversely in Iran as a three-star student. That means he has three bad marks against his name for political activism - enough to be banned from the university.

"We are not working against the system here," says fellow student Mohammad Gharib Sajadi, who has also been banned.

"The constitution has given us this right to education," he says.

"Freedom of speech is being restricted more than before in Iran," says Iran's Nobel Peace Prize winner and human rights lawyer Shirin Ebadi.

"They think students should go to their classes, read their books and then go back home and shouldn't get involved in the social and political issues around them in society - that's asking a lot!"

But President Ahmadinejad denies that his government is harassing students.

He says it has created an open atmosphere in the universities.

"The ears of the government are open to hear them," he said referring to student demands during a news conference.

Turban incident

It was the president who appointed a cleric for the first time since the revolution to head Tehran University - the country's most political and prestigious university.

Some students say the government is trying to frighten them

Mr Ahmadinejad told journalists the chancellor should be friendly with the students, moving among them and visiting their dormitories - otherwise he should give up his job to someone else.

The first time the new chancellor entered the university, students protested by knocking off his turban - a sign of extreme disrespect for a cleric.

"If I had not been well protected I would have been suffocated and there was a possibility of a crime like murder being committed," said the chancellor, Ayatollah Amid-Zanjani, after the incident.

However he added that "students have the right to protest".

The chancellor denies student allegations that there have been 17 protests against him inside Tehran University in the last year alone.

He says apart from the turban incident there was only a protest on Iranian Students' Day on 6 December, which, he said, was attended by at most 40 people.

'Cleaning the slate'

The photographs of the event showed the crowd was much bigger.

President Ahmadinejad denies harassing academics and students

And there is mobile phone footage from a demonstration in the summer at which the posters make it pretty clear what the students think of their new Ayatollah-turned-chancellor.

"This is not a religious seminary - it's a university," read one poster.

But it is not just students who are angry - professors have also faced problems.

The new chancellor forcibly retired 45 teachers from Tehran University. He said they were past the retirement age, although they were younger than him.

"The majority of the retired teachers couldn't reach the standard of a full professor after 30 years of teaching at this university. They didn't manage to do any research to improve their position," Ayatollah Amid-Zanjani said.

"It seems this is the start of a project to clean the slate - to get rid of those intellectuals who are secular opponents of the government," says student activist Abdullah Momeni.

He believes the purge started after President Ahmadinejad spoke about the need to remove secular and liberal thought from the universities.

Students complain the international community is not paying enough attention to the worsening human rights situation in Iran because of the obsession with the nuclear issue.

"The Islamic Republic has managed to focus the international community's attention on Iran's nuclear case and the possibility of an Israeli attack. That has diverted attention from the human rights situation in Iran," says Mr Nesbati.

He believes it is possible that one day Iranian officials will solve the nuclear crisis but "in the mean time they will have crushed all their internal critics".